The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine constructed for sucking evaporated fuel generated in a fuel tank into the intake system of the internal combustion engine so as to burn the evaporated fuel.
There is a conventional control apparatus (e.g., JP-A-63-129159) adapted for storing in a canister evaporated fuel generated in a fuel tank, discharging or purging from the canister the stored fuel together with air to the intake system of an internal combustion engine so as to be burnt, performing an air-fuel ratio learning operation while the fuel purging operation is stopped in order to avoid influences of the evaporated fuel gas on the learned values of air-fuel ratio.
There is another type conventional control apparatus (e.g., JP-A-2-130240) adapted for updating a learned value of air-fuel ratio by detecting per every engine-operation region a difference between an air-fuel ratio feedback value and a reference value, and subtracting from the detected difference value a value of air-fuel ratio deviation caused due to a quantity of evaporated fuel.
According to the above first type apparatus, it is necessary to frequently stop the fuel purging operation every execution of the air-fuel ratio learning operation, thus it is subjected to a problem of reduction of the fuel purging performance due to a consequent reduction of effective time period of the purging operation.
According to the above second type apparatus, it has a problem that correct air-fuel ratio learning operation can not be done since the variation of an air-fuel ratio feedback value due to influences of evaporated fuel gas becomes larger than the variation of the air-fuel ratio feedback value to be learnt, when the concentration of the evaporated fuel is high.
A preceding U.S. Pat. No. 5,299,546 entitled "Air-fuel ratio control apparatus of internal combustion engine" was filed on Apr. 27, 1993 on the basis of Japanese patent application No. 4-109592 (dated Apr. 28, 1992) and assigned to the present assignee. The U.S. application is concerned with air-fuel ratio control dependent upon fuel purge rate and evaporated fuel gas concentration and inhibition of updating of air-fuel ratio learned values for a predetermined period after start of fuel purging operation.